1. Field of the Invention
This invention relates to a heterojunction field effect transistor.
2. Description of the Prior Art
A double heterojunction FET (field effect transistor), frequently used in an amplifier for high frequency communication, having a double heterojunction structure is known. FIG. 3 is a cross-sectional view of an example of such a prior art double heterojunction FET. FIG. 2 shows gate voltage depending characteristics (gm-Vgs characteristic) with respect to a mutual conductance of the prior art double heterojunction FET shown in FIG. 3 together with heterojunction FET of this invention. This double heterojunction FET comprises a double heterojunction structure including two n-type AlGaAs layers 104 and 108 and a GaAs quantum well layer 106 sandwiched between the n-type AlGaAs layers 104 and 108. Because the GaAs quantum well layer 106 is supplied with electrons from both n-type AlGaAs layers 104 and 108, a density of electrons stored in the GaAs quantum well layer 106 is about twice that in the conventional single heterojunction structure. Moreover, a spreading area of electrons is adjustable by changing the thickness of the GaAs quantum well layer 106, so that if an appropriate thickness of the GaAs quantum well layer 106 is set, it is possible to obtain a relation between a mutual conductance and the gate voltage as shown by a curve "d" in FIG. 2 wherein the mutual conductance can be maintained to a high level over a relatively wide gate voltage range. The smaller the change in the mutual conductance around the operation point, the smaller the second order and third order distortion characteristics. Therefore, the double heterojunction structure is improved in the distortion characteristic compared with the case of the conventional single heterojunction structure when the operation point is set around the maximum value of the mutual conductance.
However, in the case of the double heterojunction structure, a decrease in the mutual conductance at a high gate voltage (a positive gate voltage) region is large, so that there is a problem that the distortion characteristic becomes poor when a voltage of the input signal to the gate is large.